Quantifying combined effects of in-plane and out-of-plane constraint in an aluminium alloy

Abstract

Fracture load of a component can depend on the geometry as well as the material properties due to the effect of constraint on the plastic region which forms around the crack tip prior to fracture. The constraint has two components: in-plane constraint, due to the size and geometry of the component, and out-of-plane constraint, mainly due to its thickness but also affected by other geometric parameters. There are currently methods to quantify both types of constraint separately as well as suggested parameters that quantify both simultaneously. However, experimental programmes that objectively compare the applicability of each are few and far between, especially on the out-of-plane constraint. In this paper we report the results of a substantial experimental programme making use of single edge notched bend (SENB) aluminium alloy specimens to study the combined effects of in-plane and out-of-plane constraint. These experiments involved testing the specimens in a three-point bending configuration with nine combinations of thickness and crack length. This allowed fracture loads to be measured then converted to stress intensity factor K by the use of three-dimensional finite element simulations. Common constraint parameters such as the Q parameter and plastic zone volume were extracted from these simulations. The results showed that K varied between 31 and 55 MPa√m for various levels of in and out-of-plane constraint. Both the magnitude of Q and the plastic zone volume increase with reducing plastic constraint. We showed that both Q and plastic zone volume could be used to quantify the combined effects of in and out-of-plane constraint, however the uncertainty associated with the plastic zone volume is less than that of Q.